A conventional microstrip antenna is generally planar and comprises an antenna element in the form of a conductive layer on one side of a dielectric substrate layer, and a conductive ground layer on the opposite side of the substrate.
Antennas of this type are advantageous in a number of applications because of their relatively low profile and ease of manufacture, as well as their compatibility with other components implemented in microstrip configurations.
In prior microstrip antennas, the propagation pattern may be substantially independent of azimuth at high elevation angles, that is, for radiation directions that do not depart greatly from the normal. At low elevations, on the other hand, the sensitivity of the same antenna may be largely dependent on azimuthal direction and, in some cases, the antenna is insensitive in all such directions at low elevations. An example of this characteristic is a circularly polarized microstrip antenna which, in prior convention configurations, has reasonable sensitivity with the desired polarization only at elevations that are relatively close to the normal.
However, for a number of applications, the antenna must function effectively over a wide range of elevation angles and, in particular, it is necessary that the antenna function at relatively low angles. An example of such an application is an antenna used with a Global Positioning System receiver. The receiver will ordinarily receive circularly polarized signals simultaneously from a number of earth-orbiting satellites having a wide range of instantaneous elevations.
Receiving antennas usually function in conjunction with signal conditioning circuit components such as phase shifters, filters, and amplifiers that may also be arranged in microstrip configurations. The present invention relates in part to an assembly of a microstrip antenna and these auxiliary circuit components in a compact form to facilitate the use of the assembly in small transportable equipment. An example of such equipment is a receiver for receiving and processing signals transmitted by the satellites in the Global Positioning System.
It is therefore an object of this invention to provide a microstrip antenna that is sensitive to circularly polarized signals over a wide range of elevation angles.
Another object is to provide an antenna assembly that has a relatively low cost and provides adequate shielding of auxiliary components from the environment.
An antenna assembly embodying the invention comprises a microstrip antenna having a dome-like configuration and a substrate, carrying the signal conditioning components associated with the antenna, attached to and closing off the interior of the dome structure. A radiative patch extends over and down from the top surface of the substrate. The signal conditioning components, which are thus disposed within the interior of the dome structure, are interconnected by microstrip conductors. The ground plane for this microstrip circuitry is connected to a metallic lining on the interior surface of the dome structure which serves as the "ground plane" for the antenna. These two ground planes thus effectively completely enclose the signal conditioning circuitry and thereby effectively shield it from the exterior of the assembly.
The dome structure preferably has a polyhedral configuration, specifically a truncated pyramid, with side surfaces facing in various azimuthal directions. The radiative patch is preferably rectangular, with corner portions extending down over the side surfaces. These corner portions are, in effect, circularly polarized sub-antennas having substantial sensitivity at low elevations. Consequently, the overall antenna exhibits good sensitivity to circularly polarized signals over a wide range of elevations and in all azimuthal directions.
This arrangement provides for a compact, easily manufactured assembly. Moreover, the assembly is rugged and the components thereof are protected from electromagnetic interference and environmental stress, which makes the assembly highly useful for a number of applications including the Global Positioning System receivers mentioned above.